1. Field of the Disclosure
The present disclosure relates to a wet-laid nonwoven fabric, and more particularly, to a wet-laid nonwoven fabric for a semipermeable membrane supporting body, which serves as a supporting body for film formation in the production of a semipermeable membrane having a separative function, such as an ultrafiltration membrane, a microfiltration membrane, or a reverse osmotic (RO) membrane, reinforces a semipermeable membrane, does not cause strike through of the semipermeable membrane coating liquid at the time of film formation, and is intended to obtain a semipermeable membrane without any coating defects.
2. Discussion of the Background Art
In recent years, the applications where semipermeable membranes are applied are increasing continuously in the fields of the removal of impurities in beverages/industrial water, desalination of sea water, the removal of saprophytic bacteria in foods, and effluent treatment, or in the biochemical field. Furthermore, the research in these fields is in rapid progress both at home and abroad.
Regarding the material for a semipermeable membrane, various polymers such as regenerated cellulose, cellulose derivatives, polyvinyl alcohol, polysulfones, and polyamides are selected in accordance with the applications. However, the strength of the membrane itself is weak, and when used alone, the membrane cannot tolerate a high pressure such as 1 MPa to 10 MPa or higher, at which the membrane is used in ultrafiltration, reverse osmosis or the like. Thus, there is a need to produce a membrane on the surface of a supporting body such as a nonwoven fabric having a high strength and high liquid permeability.
In order to obtain the required liquid permeability, tensile strength, wet strength and durability, synthetic fiber nonwoven fabrics obtained by forming synthetic fibers of polyester, polyolefin or the like into a sheet form in a wet process or a dry process, and fusion bonding the fibers by subjecting the fibers to hot press processing, are generally used as the supporting body. There is a problem in that case, in which the non-uniformity of these nonwoven fabrics brings about non-uniformity of the semipermeable membrane provided thereon, and as a result, sufficient performance may not be obtained, or the film thickness required to obtain sufficient performance may be increased, resulting in a decrease in the filtration efficiency. Therefore, a nonwoven fabric to be used as a supporting body is required to be uniform as far as possible and to be free of pinhole defects or the like.
In regard to a nonwoven fabric as a semipermeable membrane supporting body, production methods thereof are conventionally known. For example, there has been suggested a method of promoting the prevention of strike through of a semipermeable membrane coating liquid while maintaining low resistance to liquid permeation, by fabricating a structure having coarseness and fineness in the Z direction by using polyester fibers having different fiber thicknesses (see, for example, Patent Document 1).
Furthermore, there has been suggested a method of providing a nonwoven fabric which has enhanced dimensional stability when tensile stress is applied, does not cause strike through, and exhibits excellent surface smoothness and excellent adhesiveness to membranes, by using a polyester fiber having a particular thermal shrinkage stress and a particular birefringence (see, for example, Patent Document 2).
Furthermore, it is said that bending of the supporting body in the width direction at the time of coating of a semipermeable membrane causes non-uniformity of the semipermeable membrane layer. Thus, it has been suggested to form a uniform semipermeable membrane layer by controlling the orientation of fibers (see, for example, Patent Document 3).
The inventors of the present disclosure have suggested an excellent semipermeable membrane supporting body, with which a semipermeable membrane that does not suffer strike through of the coating liquid when the semipermeable membrane is formed on the supporting body, and which is free of defects with the minimum thickness required, can be obtained by minimizing the non-uniformity of the mass of the sheet in the XY direction (see, for example, Patent Document 4).
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 60-238103
Patent Document 2: JP-A No. 10-225630
Patent Document 3: JP-A No. 2002-95937
Patent Document 4: JP-A No. 2008-238147
However, it has been found that even if the methods disclosed in Patent Documents 1 to 4 are carried out, there is a defect of semipermeable membrane supporting bodies that cannot be rectified. This defect exists in a local spot-like manner on a wet-laid nonwoven fabric sheet which is a semipermeable membrane supporting body. When a semipermeable membrane coating liquid is applied on this area, permeability of the liquid partially changes, and it becomes difficult for the liquid to permeate. Thereby, there occurs a problem of coating defects such as the thickness of the semipermeable membrane at this area becomes extremely thin, or the semipermeable membrane surface becomes wrinkled.
The inventors of the present disclosure conducted a thorough investigation on this problem, and as a result, the inventors found that this defect is associated with a decrease in the sheet density, with the synthetic fibers that constitute a wet-laid nonwoven fabric being in sparse state. Furthermore, it was found that when a semipermeable membrane coating liquid is applied to a site where this density is low (hereinafter, will be described as a “low density defect”), the permeability of the liquid is different from that of other sites so that coating defects occur.
There are various causes for the occurrence of low density defects. It was found that in many cases, low density defects are generated by the production process for a wet-laid nonwoven fabric for semipermeable membrane supporting body, particularly a heat calender that performs hot press processing of the wet-laid nonwoven fabric, or a hot pressing apparatus equivalent thereto. For instance, defects such as chips, dents and attached foreign materials of a heat calender roll, particularly a metal roll, cause inhibition of compression or temperature unevenness at the time of hot press processing, and thereby inhibit thermal fusion of synthetic fibers. Some of these defects can be visually detected, but there are some defects that may be overlooked and undetected because of being in the middle of production. Furthermore, these low density defects are not easily distinguishable from other normal areas even if a wet-laid nonwoven fabric thus produced is visually observed, and the low density defects cannot be easily detected. Therefore, unless a semipermeable membrane liquid is actually applied on a wet-laid nonwoven fabric, the low density defects cannot be detected, and accordingly, it has been laborious in view of product quality management. Actually, in many cases, manufacturers who produce wet-laid nonwoven fabrics for semipermeable membrane supporting bodies may not proceed to the process of applying a semipermeable membrane liquid. Therefore, it has been a huge loss for the manufacturers that the occurrence of low density defects cannot be detected, and defective products are generated in large quantities.
Particularly, when a wet-laid nonwoven fabric is used as a semipermeable membrane supporting body, low density defects are generated significantly. The reasons for this are that since the wet-laid nonwoven fabric uses cut fibers of synthetic fibers having a relatively shorter fiber length, the fibers can be easily arranged not only in a planar direction but also in the direction of sheet thickness, and the wet-laid nonwoven fabric before being subjected to hot press processing is prone to have a lower density and to be highly bulky, and that if there are factors inhibiting the thermal fusion described above at the time of hot press processing, these low density areas are likely to remain. On the other hand, when a dry-laid nonwoven fabric is used as a semipermeable membrane supporting body, if production methods such as melt blowing and spun bonding are used, continuous long fibers are used as the synthetic fiber, the fibers can be relatively easily arranged in a planar direction, while the dry-laid nonwoven fabric does not become highly bulky. Therefore, low density defects are not easily produced.
There is a demand for a production method in which low density defects that cause defects in the semipermeable membrane coating layer are detected in the stage of producing wet-laid nonwoven fabrics, and measures are taken, and which does not generate low density defects. Furthermore, there is a demand for a wet-laid nonwoven fabric for a semipermeable membrane supporting body that is free of low density defects.
Thus, it is an object of the present disclosure to provide a uniform semipermeable membrane supporting body which does not cause defects in the semipermeable membrane coating layer when a semipermeable membrane coating liquid is applied, and which is free of low density defects. It is another object of the disclosure to provide a method for detecting low density defects.